We postulated that the development of bronchopulmonary dysplasia (BPD) is mediated by reactive changes in the gene expression of specific cytokines and consequently, by changes in the levels or the bioactivity of these cytokines. These changes markedly affect the composition of the resident and nonresident cells and of the extracellular matrix within the neonatal lung. In this study we will investigate the effect of mechanical ventilation and oxygen therapy on the immature lung of premature babies as reflected by the response of cytokines (MIF, GM-CSF, TNF-a, interleukin 8, TGF-B) and extracellularmatrix elements (PIINP, PICP, C-IV, Lam P1) and to establish the relation of this coordinated response to the development of bronchopulmonary dysplasia. We will also study the efffect of exogenous dexamethasone and of endogenous aldosterone on this coordinated response by cytokines and matrix.